The behavior of ring laser angular rate sensors, usually referred to as ring laser gyros, is well understood. Inherent in such sensors is the phenomena known as lock-in in which counter-propagating laser beams tend to lock together to a common frequency. The lock-in phenomena causes performance errors which have deleterious effects in navigational systems.
To avoid or reduce the effects of lock-in, the ring laser gyro may be biased by an alternating bias technique such as that shown and described in U.S. Pat. No. 3,373,650 issued in the name of J. E. Killpatrick. The alternating bias technique is usually referred to as dithering, and may be implemented by a variety of ways including electro-optical and mechanical schemes. Since dithering, by either of these mentioned techniques and the like, directly affects the behavior of the counter-propagating laser beams, the sensor readout signal will contain not only inertial rate information, but will also contain a signal component directly related to the dithering (alternating bias) of the sensor.
In situations where the gyro is mounted to an inertial platform or mounting base, and mechanical dithering is applied, the gyro output will consist of the sum of the base angular motion and the the gyro dither motion relative to the mounting base. This is true whether the sensor readout is mounted directly on the sensor (laser block mounted) or fixed relative to the sensor mounting base (case mounted) like that shown in the aforementioned patent.
The signal contribution in the sensor readout signal due to the alternating bias is herein referred to as the dither signal component. For low noise navigational systems, the dither signal component in the readout signal generally needs to be minimized or removed to avoid control problems, particularly in block mounted readout systems.
Prior art solutions to remove the dither signal component include, among others, notch filters. However, such notch filters generate gain and phase shift disturbances which can affect the stability of any control loops or any readout signal processing systems. Another solution utilizes a digital pulse subtraction technique as taught in U.S. Pat. No. 4,248,534, issued to Elbert.
A desirable solution is to remove the dither signal component by generating a correction signal which is substantially equivalent to the dither signal component. This latter approach is taught in U.S. Pat. No. 4,344,706 issued to Ljung et al. Ljung et al. teaches the use of a tracking circuit for tracking the clockwise and counterclockwise components of dither rotation. These dither components are subtracted from the usual readout signal which is responsive to the counter-propagating laser beams of the sensor thereby providing a corrected readout signal.
Another technique is to strip the dither signal component by an electronic dither compensator as taught in U.S. Pat. No. 4,610,543 issued to Ferriss. This patent illustrates an up/down count technique for a ring laser gyro readout mechanization in which readout output counts due to dither are subtracted from the readout output count in order to provide a corrected readout count.
The Ferriss patent also shows a closed-loop feedback method of reducing the dither component in the system readout signal. In Ferriss, a dither correction signal is subtracted from the gyro readout signal to derive a corrected sensor output signal. Closed loop correction is provided by generating the dither correction signal as a function of a dither reference signal representative of the dithering or alternating bias, and the relationship between the correction signal and the dither reference signal is controlled as a function of any dither signal component in the corrected sensor output signal.